Power Amplifier: The Key Device for Communication Applications

PublishTime： 2024-07-10 Article Source：NISSHINBO Official Website

A Power Amplifier (hereafter, PA) is used for transmission lines of radio frequency (RF) signals. Its role is to amplify data communication signals converted from RF signals to the level to which an antenna can transmit it as a electromagnetic wave without distortion.

Related regulations on electric waves from an antenna regulate usable frequencies and power levels, and requires the minimization of influence to other channels. To comply with these regulations, it is important not to distort the waves, which means not to affect other channels. The output power level in mobile phones reaches 1 W class, which is a very large current for semiconductors to deal with. In smartphones, PAs are the second battery-consuming component, following liquid crystal displays. Therefore, PAs must be as power-saving, high-efficiency as possible.

Device

The mainstream of former PAs was the MES-FET depression FET type. The depression type has a normally-on characteristic and a current flows between the drain and source when the voltage between gate and source is 0 V. Therefore, it can pinch off the current between the drain and the source by making the gate level lower than the source level. One way to use it as an amplifier is to pinch off by setting the source level over 0 V and making the gate level relatively lower than the source level. Another way is to apply a negative voltage to the gate. However, applying a negative voltage to the gate requires an external negative voltage source or a negative voltage generator circuit.

At that time, we constructed negative voltage generators with CMOS semiconductor chips and PA circuits with GaAs semiconductor chips. Then, each was combined in a single package to make our PA products.

As we talked in the history of GaAs semiconductors, we put hetero junction transistors (HBTs) into use in 2000s. A PA using the HBTs were incorporated in a front-end IC* for personal handy-phone systems (PHS). We have been brushing up the device characteristics of the HBTs in the successive development of PA.

GaAs HBT (Hetero-Junction Bipolar Transistor) in Volume 2

* front-end IC: a mixed IC including several components such as an RF switch, power amplifier, low noise amplifier, and mixer.

PA Design

High-power PAs can achieve high output power by enlarged transistors and the technology called power combining. However, the distortion characteristics and power consumption are as important characteristics as this high-power performance.

Generally, distortion characteristics are referred to by adjacent current leakage ratio (ACLR) or error vector magnitude (EVM), and the indicator of power consumption is called power-added efficiency (PAE). These ACLR, EVM and PAE have the most suitable conditions respectively, which are achieved by controlling the PA load impedance.

Figure 1. PA Block Diagram

Figure 1 shows a simple block diagram of a PA. The characteristics of the PA are controlled by impedance matching of the PA output and the load impedance matching circuit.

By optimizing the load impedance not only at the used frequency but also at the second and third harmonics, the PA can achieve high efficiency and low distortion. Figure 2 is a Smith chart showing impedance. The black points of the chart show frequency bands. The impedance of the second harmonic is close to open and that of the third harmonic is closer to short than that of second harmonic. These are achieved by connecting optimal inductors and capacitors for respective frequencies.

Figure 2. Example of Smith Chart Showing Impedance

This impedance matching requires strict accuracy, and a subtle change in capacitance and inductance influences the characteristics of the PA. PA designing needs this control like threading the eye of the needle, so designers make full use of their know-how and experience.

In order to accurately achieve such strict output loading and to simplify design and implementation by device users, modularization of PAs, including output matching circuits, is strongly demanded by the market.

Also, devices which correspond to multiple frequency bands such as mobile phones need PAs and load circuits optimized for each band. We develop and provide PA modules (PAM) that contain an HBT, load circuit and control IC in a single package.

Figure 3. PAM (PA Module)

Package

PAs consume a power of 1 W or more in a small chip, so thermal design is also important to radiate heat generated during operation outside the package. In thermal design, exposed pads in the back are essential for lead frame packages, and the paste to join the internal chip and the thickness of the chip itself affect heat dissipation. Also, to reduce heat resistance, modules using resin substrates need embedded through holes through which the backsides of the chip and package are connected.

Like this, materials of packages used for PAs must be selected in perspectives different from other products, and the accumulation of technology is highly important.

Conclusion

PAs directly treating high-frequency signal output from antennas must be compliant to each communication protocol standard following radio regulations. Furthermore, as they are required to have excellent performance, it is not an exaggeration that they are the crystallization of advanced technology. We Nisshinbo Micro Devices have over 20 years of development experience and have partially supported the communication industry by providing components that are usable and flexible.

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